The hostility of the wireless fading environment and channel variation makes the design of high rate communication systems very challenging. To this end, multiple-antenna systems have been shown to be effective in fading environments by providing significant performance improvements and achievable data rates in comparison to single antenna systems. Wireless communication systems employing multiple antennas both at the transmitter and the receiver demonstrate tremendous potential to meet the spectral efficiency requirements for next generation wireless applications.
Moreover, multiple transmit and receive antennas have become an integral part of the standards of many wireless systems such as cellular systems and wireless LANs. In particular, the recent development of UMTS Terrestrial Radio Access Network (UTRAN) and Evolved-UTRA has raised the need for multiple antenna systems to reach higher user data rates and better quality of service, thereby resulting in an improved overall throughput and better coverage. A number of proposals have discussed and concluded the need for multiple antenna systems to achieve the target spectral efficiency, throughput, and reliability of EUTRA. While these proposals have considered different modes of operation applicable to different scenarios, a basic common factor among them, however, is a feedback strategy to control the transmission rate and possibly a variation in transmission strategy.
The performance gain achieved by multiple antenna system increases when the knowledge of the channel state information (CSI) at each end, either the receiver or transmitter, is increased. Although perfect CSI is desirable, practical systems are usually built only on estimating the CSI at the receiver, and possibly feeding back some representation of the CSI to the transmitter through a feedback link, usually of limited capacity. The transmitter uses the information fed back to adapt the transmission to the estimated channel conditions.
Various beamforming schemes have been proposed for the case of multiple transmit antennas and a single receive antenna, as well as for higher rank MIMO systems, referred to as multi-rank beamforming (MRBF) systems. In MRBF systems, independent streams are transmitted along different eigenmodes of the channel resulting in high transmission rates without the need for space-time coding.
In addition to performance considerations, it is also desirable to achieve the highest possible spectral efficiencies in MIMO systems with reasonable receiver and transmitter complexity. Though space-time coding is theoretically capable of delivering very high spectral efficiencies, e.g. hundreds of megabits per second, its implementation becomes increasingly prohibitive as the bandwidth of the system increases.
A need therefore exists for an MRBF scheme that is capable of high throughput yet which can be implemented with reasonable complexity.